The present invention relates to a method and apparatus for coding/decoding an image signal to reduce the data amount of an image signal so as to transmit or file it.
In a conventional scheme for performing predictive coding of an image signal, a reference pixel is adaptively selected to improve coding efficiency, as disclosed in "DPCM picture coding with adaptive prediction", IEEE Trans. Commun. Vol. com-25, No. 11, Nov., 1977, pp. 1295-1302 (to be referred to "reference 1" hereinafter). In reference 1, a criterion for selecting a reference pixel is determined by checking level changes of coded image signals so that a reference pixel is selected to make an accurate prediction. Assume that a pixel x.sub.0 ("pixel" will be omitted hereinafter) is predicted in FIG. 2A. In this case, if the value of .vertline.x.sub.4 -x.sub.10 .vertline. is 10% or less of the range of an image signal, x.sub.4 is used for prediction. Otherwise, .vertline.x.sub.4 -x.sub.9 .vertline., .vertline.x.sub.4 -x.sub.1 .vertline., and .vertline.x.sub.4 -x.sub.2 .vertline. are measured. If .vertline.x.sub.4 -x.sub.9 .vertline. is the minimum value, x.sub.1 is used for prediction on the assumption that there is a contour extending from upper left to lower right. If .vertline.x.sub.4 -x.sub.1 .vertline. is the minimum value, x.sub.2 is used for prediction on the assumption that there is a contour extending in the vertical direction. If .vertline.x.sub.4 -x.sub.2 .vertline. is the minimum value, x.sub.3 is used for prediction on the assumption that there is a contour extending from upper right to lower left. Another coding scheme is disclosed in "Image Signal Coding Apparatus", Japanese Patent Laid-Open No. 58-13071 (reference 2), in which coding is performed in such a manner that a signal (to be referred to as an upper signal hereinafter) for coarsely designating the level of an image signal is coded, and the level of the image signal is finely specified by using the coding result. FIG. 6 shows the arrangement of the apparatus in reference 2. A signal Sp output from a shift register 31 in FIG. 6 represents a pixel coded to the final level, which corresponds to x.sub.1, x.sub.2, x.sub.3, and x.sub.4 in FIG. 2A. A signal S.sub.a1 output from a shift register 25 is an upper signal, which corresponds to y.sub.0 , y.sub.1, y.sub.2, y.sub.3, y.sub.4, y.sub.5, y.sub.6, y.sub.7, and y.sub.8 in FIG. 2B. Upper signals y.sub.0 to y.sub.8 corresponding to x.sub.0 to x.sub.8 in FIG. 2A. Assume that coding is currently performed to specify the final level of x.sub.0. Referring to FIG. 6 corresponding to reference 2, an N-level image signal predictive coder 30 predicts an image signal x (corresponding to x.sub.0 in FIG. 2A) on the basis of a coded upper signal S.sub.a1 and a coded image signal Sp, and an N-level image signal compression coder 29 performs compression coding of a prediction error signal e. Referring to FIG. 6, reference numeral 24 denotes an N.sub.1 -level conversion circuit; 26, an N.sub.1 -level signal predictive coder; 27, an N.sub.1 -level signal compression coder; and 28, a selection circuit.
In the conventional scheme in reference 1, since signals x.sub.0, x.sub.5, x.sub.6, x.sub.7, and x.sub.8, which are not coded yet, cannot be used to check a contour direction, determination of a contour becomes inaccurate, and the coding efficiency is degraded. In reference 2, compression coding of an image signal can be performed with high efficiency by using not only the coded image signal Sp but also the coded upper signal S.sub.a1. However, since the coded upper signal S.sub.a1 is constituted by a large number of pixels, complicated hardware is required.